The present invention is directed to bicycle transmissions and, more particularly, to an internal bicycle transmission that shifts gears when a driving member is in a predetermined rotational position.
Internal bicycle transmissions commonly have planet gear mechanisms. A planet gear mechanism comprises a gear ring, a sun gear, planet gears meshing with the gear ring and sun gear, and a carrier for supporting the planet gear. It is known that internal bicycle transmissions having such planet gear mechanisms are used to shift gears by selectively locking or unlocking any of the constituent elements of the planet gear mechanism with the aid of a clutch mechanism having lock pawls. The lock pawls are forcibly moved in the unlocking direction by a clutch-actuating mechanism that has a receiving hole for accepting the tips of the lock pawls.
When the lock pawls of the clutch mechanism are in a locked state and it is desired to change gears, a large force is needed to release the lock pawls at a time when the transmission is subjected to a driving force. Thus, the gear shifting operation is strenuous and difficult to perform unless the cyclist stops pedaling, thus increasing the complexity of the shifting operation.
Japanese Unexamined Patent Application 64-18792 describes a front shifting device designed to facilitate gear shifting so that gears can be shifted during pedaling. This front shifting device has a clutch pawl for switching the lock pawls between a locked position and an unlocked position. The clutch pawl is actuated by a clutch-actuating mechanism, and the lock pawls are forcibly actuated in the unlocking direction. However, the lock pawls are actuated indirectly via the clutch pawl rather than being actuated directly by the clutch-actuating mechanism. As a result, the mechanism for performing gear shifting is complicated., and the overall weight and cost of the transmission is increased.